Download Experiment 49

Dyes and Dyeing
A Practical Application of Bonding Principles
Problem Possibilities
How does the type of fabric or method deal with bonding and intermolecular forces involved in
fabric dyeing?
How does temperature dye intensity?
How does time affect dye intensity?
Research
The purpose of this activity is to introduce you to one example of the importance of molecular
structure and polarity in the commercial world. Cloth is made from fibers, with each fiber consisting
of millions of long-chain molecules. Some natural fibers, such as silk and wool, are essentially
protein molecules. Since proteins are made from amino acids, which have many polar and/or ionic
sites on them, the fibers we make from them should have a strong affinity for polar and ionic
substances. Because most dye molecules are either very polar or ionic, we would expect them to
bond quite readily to a fabric, such as wool. At the other extreme, nylon has very few polar sites so
it is very difficult to dye. In between these extremes are fabrics such as dacron and orlon, each with
a few polar sites scattered along the chain. As you might predict, these show intermediate attraction
for the various dyes. We will use the intensity of color as our measure of the attraction of fabric for
dye – the darker the color, the stronger the attraction.
In order to dye the low-polarity fabrics, it is necessary to alter their molecular structure to improve
their receptiveness to ionic and polar dye molecules. This is done through a process known as
mordanting (the term comes from a Latin word, which means ‘to bite;’ thus, we are helping the dye
to “bite into” the fabric). Mordanting involves affixing metal ions or some other ionic species (such
as tannic acid) to the polar sites of the fabric. These ions then serve to bind the dye to the fabric,
much as glue holds a rubber gasket to a metal fitting. As part of the study, you will investigate the
extent to which mordanting improves the dye-holding capability of the six different
fibers we will be testing.
The fabric samples that you will use are actually several different types of fibers arranged in a
pattern. These strips are used in the garment industry for testing and identification of the various
fabrics. From the preceding discussion and your experimental results, you should be able to decide
which end is the wool, and you will be able to compare directly the differences in dye characteristics
of the different fabrics. Solutions of the dyes will be located on hot plates around the
laboratory. The fabric samples are placed in the baths and left for several minutes.
You and your partner will divide the duties between you; in steps 1 and 4, one of you will use
Malachite Green, while the other uses Methyl Orange. In similar fashion, one person does step 2
while the other does step 3. Each of you is to keep your own record of observations, which you will
share with each other in completing the report.
1. Cotton is pure cellulose, a type of carbohydrate. As such, it is essentially a long chain of
carbon atoms with —C—O—H groups arranged along the chain.
a. Would you expect the atoms in the —C—O—H group to be linear or bent? Explain.
b. Make a sketch showing how hydrogen bonding could occur between the —C—O—H
groups on neighboring molecules.
2. Wool is a protein, as is your hair. Proteins contain many — NH groups on a long chain
of carbon atoms. What sort of attractive forces occur between proteins and dye
molecules? Explain.
3. The Introduction and Procedure mention that some dyes are polar molecules, while
others are ionic. Which would you expect to form the stronger bonds?
Materials
Apparatus
Tongs or forceps
Fabric test strips
Beaker (250-mL)
Safety goggles
Lab apron
Reagents
Dye baths:
Methyl orange
Congo red
Malachite green
Indigo
Mordant baths:
CuSO4(aq)
FeSO4(aq)
Dilute HCl(aq)
Safety
1. Dyes will stain skin just as well as they will fabric.
2. Some of the solutions contain skin irritants, such as acids or bases. Avoid contact.
3. Safety goggles and a lab apron must be worn in the lab at all times.
Procedure (Adapt to suit your needs)
Part 1
Untreated cloth
1. Direct Dying
In direct dying the ionic sites of the dye molecule attach themselves to ionic sites (carboxylic
acid and amine groups) of the fiber. A piece of test cloth is immersed in the dye bath and
maintained at near-boiling for a period of 5-10 minutes. The strip is then removed from the bath
and as much as possible of the dye solution is allowed to drain back into the bath. Excess dye is
rinsed away and the fabric is allowed to dry. Once the fabric has dried, you can test the color for
fastness against washing; cut the strip in half lengthwise and wash one of the half-strips. Dyes to
be tested by direct dying are Methyl Orange and Malachite Green (the same dyes will be
tested on mordanted cloth in Part 2).
2. Substantive Dyes
Fabrics such as cotton and rayons have polar sites but no ionic sites, so they do not bind to the
colors as well as cotton or silk. One way around this difficulty is to use substantive dyes.
These are large colloidal molecules are presumed to be bound to the hydroxyl groups of the
cellulose structure (cotton is pure cellulose, while rayon is cellulose acetate) by hydrogen bonds.
The substantive dye we will use is Congo Red.
Cut a test strip in half lengthwise. Immerse both halves in the Congo Red bath for about 10
minutes, then drain and remove the fabric strips to a warm water bath in your 250-mL beaker.
Wash in the warm water for as long as the dye is removed. Rinse one of the half-strips in very
dilute hydrochloric acid and observe the result. Rinse the acid-treated strip, then wash both with
soap.
3. Indigo: a Vat Dye
Indigo, the dye used to dye denim, is not very soluble in water, so it doesn’t rinse out easily. On
the other hand, indigo is held to the fabric only by relatively weak intermolecular forces,
primarily hydrogen bonds. This means that it will rub off fairly easily, as you probably have
observed. You may be surprised to see that the indigo bath is not dark blue, the color we
normally associate with the dye. In order to make the indigo dissolve in the water, it was
reduced in an oxidation-reduction reaction to produce a form of indigo that is water soluble.
Immerse a piece of test cloth in the Indigo bath and boil gently for about 10 minutes.
Thoroughly rinse the strip in water then let it dry. After school, take the strip home and cut it in
half lengthwise. Wash one half in a mild soap solution, and the other in the same soap solution
but with a little bleach added. (For a bathroom sink, about a capful of liquid bleach is ample.)
Part 2
Mordanted Cloth (Both partners should use the same mordanting bath; copper or iron.)
4. Two mordanting baths have been prepared for you: copper(II) sulfate, a source of Cu2+ ions; and
iron(II) sulfate, a source of Fe2+. Soak a test strip in one of the near-boiling mordanting baths for
at least 20 minutes (30 minutes is better), then wring it out over the mordanting bath. Dye the
strip using either Malachite Green or Methyl Orange, as in Part 1. Notice the difference between
the mordanted cloth and the untreated fabrics. Compare your results with someone who used the
other mordanting bath, but the same dye. [In the interest of time, your teacher may provide you
with strips of test fabric that have been mordanted with one of the above solutions.]
Cleaning Up
1. Dye baths can be diluted with water and flushed down the sink; your teacher may do this for you
since the baths are hot and stain easily.
2. The metal ion solutions should be saved for future use or disposed of by your teacher.
3. The test strips (except for those used in Part 3) should be left to dry overnight on labeled paper
towels. Your teacher may ask you to turn them in with your lab report; they are then yours to
keep.
4. Wash your hands before leaving the laboratory.